The present invention relates to methods of making absorbent cellulosic sheet in general, and more specifically to a process for making a non-compressively dewatered, impingement air dried absorbent sheet.
Methods of making paper tissue, towel, and the like are well known. Typically, such processes include conventional wet pressing and throughdry processes. Conventional wet pressing processes have certain advantages over conventional through air drying processes including: (1) lower energy costs associated with the mechanical removal of water rather than transpiration drying with hot air; (2) higher production speeds are more readily achieved with processes which utilize wet pressing to form a web; and (3) the process is relatively robust in that it does not require a highly permeable substrate. On the other hand, throughair drying processes have become the method of choice for new capital investment, particularly for producing soft, bulky, premium quality tissue and towel products.
One method of making throughdried products is disclosed in U.S. Pat. No. 5,607,551 to Farrington, Jr. et al. wherein uncreped, through dried products are described. According to the ""551 patent, a stream of an aqueous suspension of papermaking fibers is deposited onto a forming fabric and partially dewatered to a consistency of about 10 percent. The wet web is then transferred to a transfer fabric travelling at a slower speed than the forming fabric in order to impart increased stretch into the web. The web is then transferred to a throughdrying fabric where it is dried to a final consistency of about 95 percent or greater employing a vacuum of from about 3 to about 15 inches of mercury.
There is disclosed in U.S. Pat. No. 5,510,002 to Hermans et al. various throughdried, creped products. There is taught in connection with FIG. 2, for example, a throughdried/wet-pressed method of making crepe tissue wherein an aqueous suspension of papermaking fibers is deposited on a forming fabric, dewatered in a press nip between a pair of felts followed by wet straining the web on a throughair drying fabric, and throughair drying. The throughdried web is adhered to a Yankee dryer, further dried and creped to yield the final product.
Throughdried, creped products are also disclosed in the following patents: U.S. Pat. No. 3,994,771 to Morgan, Jr. et al.; U.S. Pat. No. 4,102,737 to Morton; and U.S. Pat. No. 4,529,480 to Trokhan. The processes described in these patents comprise, very generally, forming a web on a foraminous support, thermally pre-drying the web, applying the web to a Yankee dryer with a nip defined, in part, by an impression fabric and creping the product therefrom.
As noted in the above, throughdried products tend to exhibit enhanced bulk and superior tactile properties; however, conventional thermal dewatering with hot air tends to be energy intensive and requires a relatively permeable substrate. Thus, wet-press operations are preferable from an energy perspective and are more readily applied to high basis weight products and products made from furnishes containing recycle fiber which tends to form webs with less permeability than virgin fiber. However, wet press operations tend to utilize more fiber and thus are more costly on a square foot basis.
The state of the art is perhaps further understood by way of the following patents. It will be appreciated that high production rates (sheet speeds) are exceedingly important to the viability of any particular production process due to the large investment. In connection with paper manufacture, it has been suggested, for example, to employ an air foil to stabilize web transfer off of a Yankee dryer in order to maintain suitable production rates.
There is disclosed in U.S. Pat. No. 5,851,353 to Fiscus et al. a method for can drying wet webs for tissue products wherein a partially dewatered wet web is restrained between a pair of molding fabrics. The restrained wet web is processed over a plurality of can dryers, for example, from a consistency of about 40 percent to a consistency of at least about 70 percent. The sheet molding fabrics protect the web from direct contact with the can dryers and impart an impression on the web.
There is disclosed in U.S. Pat. No. 5,087,324 to Awofeso et al. a delaminated stratified paper towel. The towel includes a dense first layer of chemical fiber blend and a second layer of a bulky anfractuous fiber blend unitary with the first layer. The first and second layers enhance the rate of absorption and water holding capacity of the paper towel. The method of forming a delaminated stratified web of paper towel material includes supplying a first furnish directly to a wire and supplying a second furnish of a bulky anfractuous fiber blend directly on to the first furnish disposed on the wire. Thereafter, a web of paper towel is creped and embossed.
There is disclosed in U.S. Pat. No. 5,494,554 to Edwards et al. the formation of wet press tissue webs used for facial tissue, bath tissue, paper towels, or the like, produced by forming the wet tissue in layers in which the second formed layer has a consistency which is significantly less than the consistency of the first formed layer. The resulting improvement in web formation enables uniform debonding during dry creping which, in turn, provides a significant improvement in softness and reduction in linting. Wet pressed tissues made with the process according to the ""554 patent are internally debonded as measured by a high void volume index.
As will be appreciated from the foregoing, processes for making absorbent sheet generally incorporate two types of drying: (1) can drying where high density, low permeability can be tolerated and (2) throughdrying which requires a permeable substrate. The present invention is directed to making high bulk products wherein the permeability of the substrate is not critical.
There is provided in one aspect of the present invention a method of making absorbent sheet including the steps of: (a) depositing an aqueous furnish comprising cellulosic fiber on a foraminous support; (b) dewatering (preferably non-compressively dewatering) the wet web to a consistency of from about 15 to about 40 percent; (c) transferring the dewatered web at the aforesaid consistency to another fabric traveling at a speed of from about 10 to about 80 percent slower than the speed of the web prior to transfer; (d) macroscopically rearranging the web to conform to the shape of an impression fabric; and (e) impingement air drying the web to form an absorbent sheet. Typically, the web is dewatered to a consistency of from about 20 to about 30 percent prior to transfer and impingement air dried at a rate of from about 25-50 lbs of water removed per hour per square foot of drying area. Drying rates of from about 30-40 lbs/hr-ft2 are typical, over drying lengths of from about 50 to 300 feet. Impingement air drying lengths are typically from about 75 to about 200 feet, with from about 100 to 150 feet being a preferred construction of a paper machine to practice the present invention.
Most typically, the step of impingement air drying is carried out over a plurality of impingement air dryers including rotating cylinders and drying hoods sequentially arranged in a row opposing a row of reversing vacuum cylinders over which the web travels. In this arrangement, impingement exhaust air from a downline dryer can be cascaded backward to an upline dryer operating at higher humidity.
A product of any typical basis weight may be made by way of the present invention, suitably having a weight of at least 10 lbs/3000 ft2. Higher basis weight products, having basis weights of at least 15 lbs/3000 ft2 or at least 20 lbs/3000 ft2 may also be produced as will readily be appreciated from the discussion which follows.
Typically, the web is impingement air dried to a consistency of at least about 90% and in preferred embodiments to a consistency of about 95 percent or so.
In another aspect of the present invention, there is provided the additional steps of: adhering the impingement air dried web to a rotating cylinder and creping the web from the cylinder. A creping adhesive may be used, and the cylinder may be heated if so desired.
There is provided in still yet another aspect of the present invention a method of making an absorbent sheet including the steps of: (a) depositing an aqueous furnish comprising cellulosic fiber on a forming fabric; (b) dewatering the wet web to a consistency of from about 15 to about 40%; (c) transferring the dewatered web from the forming fabric to a transfer fabric traveling at a speed of from about 10 to about 80% slower than the forming fabric; (d) transferring the web to an impression fabric whereby the web is macroscopically rearranged to conform to the surface of the impression fabric; and (e) impingement air drying the web. Typically, the wet web is dewatered to a consistency of from about 20 to about 30% in step (b). So also, the transfer fabric is typically traveling at a speed of from about 15 to about 40% slower than the forming fabric.
Any suitable aqueous furnish may be employed; in many embodiments the furnish includes recycled fiber. Recycled fiber may be present in any amount;
particularly preferred embodiments oftentimes include at least about 50 percent by weight recycled fiber, based on the amount of fiber present. More than about 75 percent by weight of the fiber may be recycled fiber or the cellulosic fiber in the furnish may consist entirely of recycled fiber.
In order to achieve enhanced bulk and softness it may be desirable in many embodiments to subject at least a portion of the fiber to a curling process. For example, one may subject at least about 10 percent of the fiber in the aqueous furnish to a curling process or at least about 25 percent of the fiber in the furnish to a curling process. Where particularly high bulk is desired, one may subject 75%, 90% or even more of the fiber present in the aqueous furnish to a curling process. While any suitable curling process may be used to increase the curl inherent in the fiber, a particularly preferred process includes concurrently heat treating and convolving the fiber at an elevated temperature. Such processes may be carried out in a disk refiner, for example, with saturated steam at a pressure of from about 5 to about 150 psig. Another method of increasing the bulk may include foam forming the furnish on the forming fabric as is known in the art. See, for example, U.S. Pat. No. 5,200,035, the disclosure of which is incorporated herein by reference.
In a typical embodiment, the aqueous furnish will further include a debonding agent, such as a cationic debonding agent. In some embodiments, it may be preferred to include both a cationic debonding agent and a non-ionic surfactant.
It is desirable to dry the web at the highest rate achievable with the impingement air dryer. Preferably a drying rate of at least about 30 pounds of water removed per square foot of impingement air drying surface per hour is preferred. More preferably, a drying rate of at least 40 pounds of water removed per square foot of impingement air drying surface per hour is attained.
The present invention further includes absorbent sheet made by the aforesaid process.